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US polyphase

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240v is standard for the USA but only one phase is used and the transformer center tap is earthed to ground making it safer.
Does that mean if I create a stable clock as a reference source and check my neighboors phase I should depending on how the lines are tapped locally see that some of us are out of phase? I'm speaking of the homes total 240V supply, If I check three houses and they're tapped on separate branches I should be able to determine which one is L1 L2 L3?

Is our power deliver really that screwed up? Do they send power co folks out to adjust what houses are on what branches to balance the main three phase branches?


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The situation in England is that the mains power is 230 V to ground, 400 V between phases.

Single phase loads, such as houses, are distributed between phases, usually in rotation so that every third load is on the same phase. Where there are overhead wires, the neutral is usually the bottom one, and you can see one house connected to the top wire and the bottom one, the next house connected between the second and the bottom one, and the third between the lowest two wires. That cycle continues all along the street.

Uneven phase loads do not matter unless they get too large.


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Does that mean if I create a stable clock as a reference source and check my neighbors phase I should depending on how the lines are tapped locally see that some of us are out of phase?
Yep, that is what it means. Typically in residential areas the power company comes in with about 7,200 volts 3 phase. Some above ground on poles and some areas using buried cable. The transformers step it down to 240 volts split phase for distribution to the customers. Generally the transformers are rated between 16 KVA and 100 KVA depending on the areas and number of customers to feed.

This is not always true. In a very rural area the power company may only run a single line of three phase up for example a long country road with only a few customers. In cases like this all the customers on that line would be in phase with each other. Not unusual in areas like this to see a single small 16 KVA transformer feed a single customer (about 60 amp service).

I live in a mixed residential / commercial area, older with poles for power distribution. The upper lines on the poles are 13,300 volts and the lower lines are 7,200 volts. Walking down the street, the transformers look to be around 75 KVA and each feeds 2 to 4 residences (depending on distances between houses). Each transformers primary is tied to a different phase of the 7,200 volt lines. So absolutely if my transformer I am on is tied to one phase of the 7,200 volts and my next door neighbor's transformer is tied to a different phase of the 7,200 volts our house voltages will be 120 degrees out of phase from each other. If we share the same transformer on the pole then we are in phase with each other.



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The main difference in England is that the transformers are almost always 3-phase.

When I was involved in commercial supplies, the 400 V three phase distribution boards simply had positions 1, 4, 7 etc on red, 2, 5, 8 etc on yellow, and 3, 6, 9 etc on blue. The positions were filled with either single breakers for single phase loads, or 3 phase breakers that took 3 adjacent slots. If you had mainly single phase loads, working from one end meant that the load was spread.

In the USA, 240 V with 120 V on each leg to ground is also common, so two phase distribution boards are also available, where odd numbered circuits are on one leg and even numbered are on the other leg.

2 phase, or centre tapped to ground, is hardly ever used in the UK, except for portable power tools uses outside, where 110V centre tapped is used. That is only 55 V to ground, to minimise the danger of electric shocks.


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In the USA the house voltages are mostly 220 or 240 volts with a center tap.
Basically two phases are used. 120 V - Neutral - 120 V. ( Phase angle 180° )
Light loads like lamps, radio's, heaters etc. are connected between one phase and the neutral which is also grounded.

Larger loads like ranges, dryers, hot water systems are connected between the two phases and run on 220 or 240 volts
In a three phase situation 120 / 208 Volts systems are in use ( Phase angle 120° )

In commercial situations 277/480 Volts is used ( Phase angle 120° )

For higher loads > 150 kVA usually a High Voltage 7.2 kV or 13.3 kV transformer is fitted on site. it also depends on which state you are in as well.

For balancing purposes ideally each different house ( or group of houses fed from a transformer ) should be fed from one of the three phases of the HV system as Reloadron explains.
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Gary B

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Of course, poly-phase feeds can cause problems if one is not careful. I had a telephone technician blow two computers and interface boards before he called for help. It turned out that the outlets in the room were on different phases. The telephone switch was on one and the outlet he plugged his service PC into was on another causing a 208 VAC differential between the two hot sides instead of the expected 0 VAC. This was in Manhattan but it could have been almost anywhere.
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